1. Field of the Invention
The present invention relates to an optical recording medium for use in an optical memory apparatus, and more particularly to a phase-change type information recording medium in which a phase change is caused in a recording material of a recording layer by the application of a light beam thereto, thereby performing the recording and reproducing of information, and enabling the overwriting of information.
2. Discussion of Background
A phase-change type optical information recording medium utilizing a phase change between a crystalline phase and an amorphous phase or between one crystalline phase and another crystalline phase is conventionally known as one of the optical memory media which are capable of recording, reproducing and erasing information by the application thereto of electro-magnetic waves, such as a laser beam. This kind of phase-change type optical information recording medium enables the overwriting of information by use of a single laser beam, although such overwriting is difficult to conduct by the application of a single laser beam in magneto-optical memory using a magneto-optical recording medium.
An optical system of a drive unit for the phase-change type optical information recording medium can be designed simpler than that for a magneto-optical recording medium, so that recently research and development of this kind of recording medium has been actively made.
As disclosed in U.S. Pat. No. 3,530,441, the so-called chalcogen-based alloys, such as Ge--Te, Ge--Te--Sb--S, Ge--Te--S, Ge--Se--S, Ge--Se--Sb, Ge--As--Se, In--Te, Se--Te and Se--As, are conventionally used as recording materials for the phase-change type optical information recording medium. In addition, it is proposed to add an element of Au to the above-mentioned Ge--Te based alloy to improve the stability and to increase the rate of crystallization of the recording material as disclosed in Japanese Laid-Open Patent Application 61-219692. Furthermore, the addition of Sn and Au to the Ge--Te based alloy, and the addition of Pd to the same are respectively proposed in Japanese Laid-Open Patent Applications 61-270190 and 62-19490 for the same reasons as mentioned above. Furthermore, in Japanese Laid-Open Patent Application 62-73438, a recording material composed of a Ge--Te--Se--Sb alloy with a specific composition ratio of each component in terms of atomic percentage is proposed for the improvement of the repetition properties of recording and erasing.
However, none of the above-mentioned conventional phase-change type optical information recording media satisfies all the requirements for the phase-change type rewritable optical memory medium.
Japanese Laid-Open Patent Application 63-251290 discloses an optical information recording medium which comprises a recording layer comprising a compound of a multi-component system composed of substantially three or more components in a single crystalline phase. The single crystalline phase of the compound of a multi-component system composed of substantially three or more components is considered to contain a compound with a stoichiometric composition, for example, In.sub.3 SbTe.sub.2, in the recording layer in an amount of 90 atm.% or more. It is mentioned that by the provision of this kind of recording layer, high-speed recording and high-speed erasing can be performed to some extent. However, this optical information recording medium has the shortcomings that the erasability is low because of insufficient laser power for recording and erasing, so that erasing becomes imperfect.
Furthermore, Japanese Laid-Open Patent Application 1-277338 discloses an optical recording medium which comprises a recording layer comprising an alloy with a composition represented by the formula of (Sb.sub.a Te.sub.1-a).sub.1-b M.sub.b, wherein 0.4.ltoreq.a&lt;0.7, and b.ltoreq.0.2, and M is one element selected from the group consisting of Ag, Al, As, Au, Bi, Cu, Ga, Ge, In, Pb, Pt, Se, Si, Sn and Zn. The basic system of the aforementioned alloy is Sb.sub.2 Te.sub.3, and the addition of a large excess of Sb to this composition in terms of atomic percentage enables high-speed erasing operation and improves the repetition properties. The addition of the element M can further enhance the high-speed erasing performance. In addition to the above advantages, this reference asserts that the erasability by the application of DC light is increased. However, this reference does not show any specific erasability attained at the overwriting, and according to the experiments conducted by the inventors of the present invention, the erasability in the course of the overwriting operation is unsatisfactory and the recording sensitivity is insufficient for use in practice.
Japanese Laid-Open Patent Application 60-177446 discloses an optical recording medium which comprises a recording layer comprising an alloy with a composition represented by the formula of (In.sub.1-X Sb.sub.X).sub.1-Y M.sub.Y, wherein 0.55 .ltoreq.X.ltoreq.0.80, 0.ltoreq.Y.ltoreq.0.20, and M is one element selected from the group consisting of Au, Ag, Cu, Pd, Pt, Al, Si, Ge, Ga, Sn, Te, Se and Bi. In addition, Japanese Laid-Open Patent Application 63-228433 discloses a recording layer of an optical recording medium, which comprises an alloy with a composition of GeTe-Sb.sub.2 Te.sub.3 --Sb (excess). The sensitivity and erasability required for the recording medium cannot be satisfied in any of the above-mentioned conventional optical recording media.
Under such circumstances, the most important problems to be solved in the conventional optical recording media are to improve the recording sensitivity and erasing sensitivity, to prevent the decrease of the erasability in the course of the overwriting operation, which is caused by the remaining of unerased portions, and to extend the life of the recorded portions and unrecorded portions in the recording media.
A conventional recording material of an alloy AgInTeSb enables high-speed recording and attains the improvement of the erasability, but the repetition properties in the course of the overwriting are not sufficient for use in practice.
The above-mentioned drawbacks stem from the segregation and movement of the elements in the recording layer, which are caused by the repetition of recording and erasing operations with the application of a laser beam thereto.